A flash memory is a non-volatile storage device that can be electrically erased and reprogrammed. Flash memories are commonly used in microcontrollers and in memory cards, USB flash drives and solid-state drives for general storage and transfer of data between computers and other digital products. Flash memory devices typically store information in an array of memory cells made using floating gate transistors.
A floating gate transistor is a field effect transistor having a structure similar to a conventional metal oxide semiconductor field effect transistor (MOSFET). Floating gate MOSFETs are distinguished from conventional MOSFETs because the floating gate MOSFET transistor is a split gate transistor that includes two gates instead of one. Split gate embedded flash memory devices are widely favored because of their compatibility with complementary metal oxide semiconductor (CMOS) technology. In addition to an upper control gate, a split gate transistor includes an additional floating gate beneath the control gate and above the transistor channel but completely electrically isolated by an insulating layer such as an oxide layer that completely surrounds the floating gate. This electrically isolated floating gate creates a floating node in direct current (DC) operation with a number of inputs or secondary gates such as the control gate, formed above the floating gate and electrically isolated from it. These secondary gates or inputs are only capacitively connected to the floating gate as the floating gate is completely surrounded by highly resistive material, i.e. the insulating layer. Any charge placed on the floating gate is trapped there and the floating gate remains unchanged for long periods of time until the floating gate MOSFET is erased. Unless erased, the floating gate will not discharge for many years under normal conditions. Fowler-Nordheim Tunneling or other Hot-Carrier injection mechanisms may be used to modify the amount of charge stored in the floating gate, e.g. to erase the floating gate. The programming and erase operations are therefore critical to the operation of floating gate transistors.
The integrity of the control gate and the floating gate as well as the integrity of the insulating layer between the control gate and the floating gate, is very important because of the aforementioned electrical considerations. The electrical considerations also render the charge storage ability of the floating gate and the alignment between the control gate and the floating gate, similarly important. The floating gate, the control gate and the oxide or other dielectric layer that isolates the floating gate from the control gate, should advantageously be of high integrity and defect-free and aligned accurately with respect to one another.